Alternative pre-mRNA splicing is a common mechanism for regulating gene expression in metazoans. Indeed, RNA processing is a conduit by which genomic information is transferred to proteomic information. In fact, it is now recognized that 40-50% of the known human and mouse disease gene mutations affect the splicing process. Thus, understanding how introns are recognized and how patterns of alternative splicing are set up may allow therapeutic intervention. The overall objective of the research proposed in this grant is to elucidate biochemical mechanisms used to regulate splicing of messenger RNA precursors (pre-mRNA) during development in the fruit fly,Drosophila melanogaster. Our efforts will be focused on understanding two examples of alternative splicing in detail and, more generally, the role that RNA binding proteins play in controlling splicing patterns. In order to accomplish our overall objectives, we will: 1. Purify and analyze nuclear pre-mRNPs that contain the Drosophila hnRNP proteins, PSI, hrp48, hrp40, hrp38 and hrp36. 2. Investigate the splicing regulation of the P element and squid/hrp40 pre-mRNAs using biochemical studies. 3. Analyze the genome-wide effects on alternative pre-mRNA splicing patterns after removal of specific RNA binding splicing factors using Drosophila splice junction microarrays. 4. Analyze the structure and function of PSI, Drosophila U2AF and U2AF-related proteins.